Recuperator and method of making

ABSTRACT

Recuperator having unusually high heat transfer capability is made up of casing with headers, and matrix having tubes extending between headers and fins integral with tubes. In preferred form a continuous web of metal foil, such as stainless steel, is repeatedly folded on itself to form multiple spaced corrugations extending from one side of web. Each corrugation is pinched together at its open edge and seam welded to produce tube integral with web. Web is then wrapped on itself in spiral arrangement to produce matrix of multiplicity of very small parallel tubes. The tubes of each wrap are in contact with the web of the adjacent wrap but not secured thereto. All portions brace each other, and integral fins provide maximum heat transfer. Tubes are rigidly connected only to headers, and mid portion of matrix is free to work in response to temperature gradients, greatly reducing localized stresses.

United States Patent [191 Troy 1 Jan. 1,1974

[ RECUPERATOR AND METHOD OF MAKING [75] Inventor: Walter C. Troy,National City,

Calif.

[73] Assignee: Rohr Corporation, Chula Vista,

Calif.

[22] Filed: Oct. 26, 1971 [21] App]. No.: 191,998

[52] US. Cl 165/165, 165/158, 165/160, 165/165, 165/183, 60/3951 R,29/l57.3 R,

[51 Int. Cl. F28d 9/00 [58] Field of Search 165/183, 175, 160,

165/165, 166, 158, 122, 171, 148; 60/3951 R; 29/1573 R, 157.3 D, 157.3C; 113/118 R, 118 D, 118 C; 432/179 371,608 4/1932 Great Britain 165/183Primary Examiner-Albert W. Davis, Jr. Attorney-George E. Pearson [57]ABSTRACT Recuperator having unusually high heat transfer capability ismade up of casing with headers, and matrix having tubes extendingbetween headers and fins integral with tubes. In preferred form acontinuous web of metal foil, such as stainless steel, is repeatedlyfolded on itself to form multiple spaced corrugations extending from oneside of web. Each corrugation is pinched together at its open edge andseam welded to produce tube integral with web. Web is then wrapped onitself in spiral arrangement to produce matrix of multiplicity of verysmall parallel tubes. The tubes of each wrap are in contact with the webof the adjacent wrap but not secured thereto. All portions brace eachother, and integral fins provide maximum heat transfer. Tubes arerigidly connected only to headers, and mid portion of matrix is free towork in response to temperature gradients, greatly reducing localizedstresses.

4 Claims, 12 Drawing Figures INLET AIR PATENTEDJAH 1153;: 3,782,457

SHEET 2 OF 3 INLET AIR FIG. 7

FIGQQ PATEHTEU 5 4 3,782,457.

SHEET 3 [If 3 GAS IN M;

FIG. '1'] GAS OUT N RECUPERATOR AND METHOD OF MAKING BACKGROUND OF THEINVENTION This invention lies in the field of recuperators, or heatexchangers, of the general type used in connection with gas turbines torecover waste heat from the exhaust gas and transfer it to the incomingair, although it is not limited to this type of equipment. It isdirected particularly to a recuperator which is light and durable whileachieving very high heat transfer efficiency in terms of unit volume orunit weight.

The exhaust gas from the turbines of aircraft jet engines mustnecessarily be expelled directly to atmosphere since it is the source ofthrust. However, many gas turbines in small size ranges deliver theirpower through mechanical shaft means to operate land vehicles, electricpower generators, and other devices, and it is common to recover energyfrom their exhaust gases by passing them through heat exchangers,commonly referred to as recuperators, in which incoming air for theturbines is passed in heat exchange relation with the exhaust gases.

The recuperators commonly used for this purpose comprise casings throughwhich a plurality of tubes pass to serve as flow paths for the exhaustgas. Incoming air is passed through such casing and around the tubes toextract heat from the exhaust gas. Heat transfer is increased by one ormore schemes such as providing multiple fins brazed on the tubes, orfin-like framework within the casing holding the tubes in place andincreasing the heat transfer area, or providing tortuous air flow pathsback and forth over the tubes. All of these schemes are helpful but theefficiency is still low in terms of weight and volume. The tubes areusually relatively large so that a great deal of gas is transmittedwithout giving up much heat, and the tubes are widely spaced withrespect to their diameters so that the volume of the casing is notefficiently used. Also, if the tubes are supported by framework, theheat flow paths to the remote parts of the framework are long andrelatively ineffective. If the tubes, fins, and framework are brazedtogether, corrosion problems arise and high temperature stresses resultfrom uneven heat transfer.

Attempts have been made from time to time to produce a more unitary heatexchanger by uniting two sheets of metal with weld lines which definetube outlines between them, and then applying internal pressure betweenthe weld lines to expand the areas into tube-like formations. This is avery difficult and expensive operation and does not produce very uniformresults. In addition, the structure must then be curved or wrapped intosome sort of annular configuration, and this is very difficult andunsatifactory because of the great stiffness of the welded structure.Moreover, the double walls use up space, add weight, and reduce the rateof heat transfer.

SUMMARY OF THE INVENTION The present invention overcomes thedifficulties mentioned above and provides a recuperator which is verycompact and light and operates at an efficiency far higher than previoussuch devices. Moreover it is easy to manufacture and very durable inservice.

Generally stated, and in presently preferred forms, the heart of thedevice is the core material for the recuperator matrix. This corematerial comprises an elongate web of metal foil, such as stainlesssteel, only a few thousandths of an inch thick which is repeatedlyfolded back and forth on itself to form a multiplicity of corrugationsprotruding from only one side of the general plane of the web. Thecorrugations are parallel to each other and to the web and are ratherclosely spaced, and they extend from edge to edge of the web laterallyof its principal axis. The open edges of the corrugations are thenpinched together and welded along lines parallel to the web to make theminto a multiplicity of tubes, all of which are integral with the web.The entire web is readily bendable about its lateral axes because of theflexibility of the thin foil material.

The web is then wrapped tightly on itself in a spiral arrangement abouta small core or a large hollow core depending on its intended use untila cylindrical matrix is produced in which the wraps or layers areradially overlaid and the tubes of one layer are in contact with the webof an adjacent layer although not bonded thereto. Headers are thenapplied to each end of the matrix by any suitable means, such aselectrical discharge machining, powder metallurgy, or ceramic molding.Casing walls are connected to the header to complete the unit.

Air inlets and outlets are formed in the casing walls near the ends andradial flow paths are formed by providing apertures in some or all ofthe web portions between the tubes and adjacent to the ends of thematrix. A longitudinal air flow path through the matrix is defined bythe longitudinal spaces between the tubes and the web.

While the spiral wrap configuration is the most preferred form atpresent, it is perfectly feasible to divide the elongate web into shortlengths determined by the shape of the casing to be used, and to stackthe web sections or layers in tube to web relation and achieve the samegoal of compact and light weight construction.

BRIEF DESCRIPTION OF THE DRAWINGS Various other advantages and featuresof novelty will become apparent as the description proceeds inconjunction with the accompanying drawings, in which:

FIG. I is a schematic perspective view of an elongate web of foilmaterial, part of which has been corrugated;

FIG. 2 is a view similar to FIG. 1 after a second forming step;

FIG. 3 is a view similar to FIG. I after a third forming step;

FIG. 4 is a view similar to FIG. 1 after a fourth forming step;

FIG. 5 is a view similar to FIG. 4 showing a modificatron;

FIG. 6 is a schematic end view of a matrix spirally wrapped on a minimumcoref FIG. 7 is a view similar to FIG. 6 but with a large hollow core toreceive a component;

FIG. 8 is a schematic perspective view of a recupera tor with. thematrix of FIG. 7 with portions of the sidewalls removed;

FIG. 9 is a schematic plan view of a portion of the web used to make upthe matrix of FIGS. 7 and 8;

FIG. 10 is a schematic sectional view of the matrix and headers of FIG.8;

FIG. 11 is a view similar to FIG. 10 with the matrix wrapped on aminimum core; and

FIG. 12 is a schematic cross sectional view of a moditied form ofrecuperator.

DESCRIPTION OF PREFERRED EMBODIMENTS The manner of producing corematerial for use in making up the matrix is illustrated schematically inFIGS. 1 to 5. The relative sizes of components in these and the otherfigures are modified for ease of illustration and description. It willbe seen in FIG. 1 that the starting material for the matrix is .a verylong continuous flat length or web of metal foil, such as stainlesssteel, which is only a few thousandths of an inch thick. The first stepis to process the web in any suitable machine to form a multiplicity ofreversely bent portions or corrugations 12 which are generally U-shapedand project only from one face of the web. The corrugations extend inspaced parallel relation laterally of the principal axis of the web.

In the next phase, the open edges 14 of the corrugations are pinchedtogether and welded from end to end along lines parallel to the plane ofthe web to, produce individual sealed tubes 16 or tear dropconfiguration as seen in FIG. 2. .A series of slots or apertures 18 arethen pierced in some or all of the web portions 20 between the tubes andinward of the side edges 22 of the web, as seen in FIG. 3. Typically,the slots may be about A to inch in length with lands of about l/l6 inchbetween them to maintain the web strength. For maximum flow capacity asizing tool is then passed through each tube to give it the cylindricalformation shown in FIG. 4. The core material is now completed and readyto be wrapped around a mandrel or core in a spiral arrangement.

The full width of the web, as in FIG. 4, is retained for applyingheaders to the spirally wrapped matrix by means of electrical dischargemachining in the well known manner. However, when other systems are usedfor forming the headers in situ, as by powder metallurgy or ceramicmolding, the side edges 22 together with the neck members 24 whichconnect the tubes to the web are cut back as shown in FIG. 5 to form newside edges 26, with the tube ends protruding a predetermined distance.

In order to make up a matrix, the web may be readily bent about lateralaxes because of the flexibility of the very thin foil and wrapped inspiral arrangement about a mandrel 28 provided with a cam-like formation30 to initiate the spiral form as indicated schematically in FIG. 6. Thewrapping is continued with the tubes of one wrap or layer in contactwith the web of the adjacent wrap or layer until a matrix 32 of thedesired capacity is attained. Headers are then applied in any suitablemanner so that the tube ends pass through the headers and arepermanently secured thereto, and then the casing is applied to completethe closure. Since all of the layers are in contact, they are mutuallyself supporting. At the same time, only the tube ends are held rigidlyand the remainder of the matrix may work in response to high temperaturegradients because the layers are not rigidly secured to each other, andthe stresses of uneven heating are substantially eliminated.

When it is desired to form the matrix as a muff to surround some othercomponent such as the combustor of a jet engine, the same process isused starting with a hollow mandrel 34 large enough to encompass thedesired component and provided with a spiral initiating formation 36.Again the wrapping is continued until a matrix 38 of the desired size isattained, and headers and casing are applied in any suitable way.

A complete recuperator incorporating the muff type matrix of FIG. 7 isschematically illustrated in FIG. 8, where it will be seen that headers40 and 42 are permanently secured to the tube ends and a casing wall 4surrounds the matrix between the headers. The end portions of the casingwall have been omitted for clarity of illustration. Air enters thecasing at the end adjacent to header 42 and flows radially inwardthrough the slots or apertures 18 to all portions of the cross section.

It then flows longitudinally through all of the passageways definedbetween the tubes and the web, and at the end adjacent to header 40 itflows radially inward through slots or apertures 18 to the interior ofthe core where it may be used as the combustion air for the turbine. Theexhaust gas from the turbine flows through tubes 16 from header 40 toheader 42. Thus this arrangement produces a combination of lateral flowand counter flow to achieve maximum heat transfer. The effectiveness is,of course, greatly enhanced by the unusually large number of tubes andthe large area of integral fins made possible by the presentconstruction. A typical example is a recuperator configuredapproximately like FIG. 8 having an inside diameter of about 10 inchesand an outside diameter of about 20 inches and a length of about 13inches, and containing upward of 22,000 tubes each having an outsidediameter of about 0.078 inch.

It is desirable in most recuperator constructions in which air entersand leaves the casing radially to provide the greatest area of flow pathadjacent to the inlet and outlet and to gradually decrease the area to aminimum at the wall remote from the entrance and exit in order toimprove distribution through the various portions of the matrix. In thepresent construction, this may be accomplished in the manner indicatedin FIG. 9, in which a web 10 with its integral tubes not shown isprovided with apertures 18 extending laterally inward from adjacent themargins or side edges 22. At one end of the web, their extent is apredetermined minimum and at the other end their extent is apredetermined maximum, which the extent gradually increasing along thelength of the web. The web shown in this figure is suitable for makingup the matrix for the recuperator of FIG. 8. If it is wrapped fromeither end of the web the resulting matrix will have maximum extentslots at the radially inner side of one end and at the radially outerside of the opposite end as in FIG. 8. The result is more graphicallyshown in FIG. 10 which represents the matrix formed by spirally wrappingthe web of FIG. 9.

The same scheme may be used to produce the matrix 32 having a minimumcore. In such case, both the inlet and the outlet are at the radiallyouter side of the matrix, and therefore the slot arrangement must bemodified so that the minimum slot extent at both margins is at one endof the web and the maximum slot extent is at the other end. The minimumslot end of the web must be located at the core so that the smallestflow path areas will be at the center of the matrix. When this is done,the resulting matrix will be as indicated in FIG. I 1.

While the spiral wrap arrangement is the most preferred for general use,there are various special installations where some modifications isdesirable. In the example shown in FIG. 12, the total web is cut intoindividual web sections 46 of predetermined length and the sections arethen stacked in multiple layers to produce of efficiency, weight,volume, and durability because it is extremely compact and heat transferis multiplied manyfold since it is practical to use an unusually largenumber of tubes in a given space with adequate finning and mutualreinforcement. I

While the invention has been described in detail in its presentpreferred embodiment, it will be obvious to those skilled in the art,after understanding this invention, that various changes andmodifications may be made therein, specifically in the fabrication ofthe core material, without departing from the spirit or scope thereof.

Having thus described the invention, what is claimed as new and usefuland is desired to be protected by US. Letters Patent is:

l. A recuperator comprising: a casing having side walls and a header ateach end, and a heat exchanger matrix located within the casing; thematrix comprising a plurality of layers of heat exchanging core materialarranged in overlying relation with each other; the core materialconsisting of metal foil web means provided with a multiplicity ofelongate tubes integrally formed from the web means and arranged at oneside of the web means in closely spaced parallel relation with eachother and parallel with the web means; the tubes of one layer being incontact with the web means of the adjacent layer while remaining freeand unsecured thereto whereby the tubes and web means are free to moverelative to each other when subjected to thermal stresses produced inthe respective members; the ends of the tubes extending through andbeing secured to their associated headers; the tubes providing a flowpath through the headers and the length of the casing for the flow of afirst heat exchanging fluid; fluid inlet and outlet means formed in thecasing; and the spaces between the tubes, the web means, and the casingwalls providing a flowpath through the interior of the casing betweenthe inlet and outlet means for the flow of a second heat exchangingfluid.

2. A recuperator as claimed in claim 1; the layers of core materialbeing arranged generally circumferentially about the longitudinal axisof the casing with the tubes being directed parallel to the longitudinalaxis and spaced peripherally and radially from each other.

3. A recuperator as claimed in claim 2; the core material comprisingcontinuous web means wrapped upon itself in a spiral arrangement.

4. A recuperator as claimed in claim 1; the core material being dividedinto a plurality of web sections to define discrete layers.

1. A recuperator comprising: a casing having side walls and a header ateach end, and a heat exchanger matrix located within the casing; thematrix comprising a plurality of layers of heat exchanging core materialarranged in overlying relation with each other; the core materialconsisting of metal foil web means provided with a multiplicity ofelongate tubes integrally formed from the web means and arranged at oneside of the web means in closely spaced parallel relation with eachother and parallel with the web means; the tubes of one layer being incontact with the web means of the adjacent layer while remaining freeand unsecured thereto whereby the tubes and web means are free to moverelative to each other when subjected to thermal stresses produced inthe respective members; the ends of the tubes extending through andbeing secured to their associated headers; the tubes providing a flowpath through the headers and the length of the casing for the flow of afirst heat exchanging fluid; fluid inlet and outlet means formed in thecasing; and the spaces between the tubes, the web means, and the casingwalls providing a flow path through the interior of the casing betweenthe inlet and outlet means for the flow of a second heat exchangingfluid.
 2. A recuperator as claimed in claim 1; the layers of corematerial being arranged generally circumferentially about thelongitudinal axis of the casing with the tubes being directed parallelto the longitudinal axis and spaced peripherally and radially from eachother.
 3. A recuperator as claimed in claim 2; the core materialcomprising continuous web means wrapped upon itself in a spiralarrangement.
 4. A recuperator as claimed in claim 1; the core materialbeing divided into a plurality of web sections to define discretelayers.